Lanthionine bridged peptides

ABSTRACT

Disclosed are lanthionine bridged peptides having the structure  
                 
 
     methods of their preparation and their use as pharmacologically active agents.

[0001] This application is a Continuation of Ser. No. 09/384,601, filed on Aug. 26, 1999, now allowed, which in turn was a Continuation of Ser. No. 08/467,472 filed Jun. 6, 1995, now abandoned which in turn was a Continuation-in-Part of Ser. No. 08/021,606 filed Jan. 28, 1993, now abandoned, which in turn was a Continuation of Ser. No. 07/742,908 filed Aug. 9, 1991, now abandoned.

[0002] It is a basic goal in peptide chemistry to design molecules for medical or industrial application. Design means that naturally occurring peptides which have a biological different activity are modified in order to obtain molecules which have advantages over the naturally occurring peptides in different respects. There are several groups of peptides which act as hormones, as neurotoxins or as plant regulating agents. These peptides are usually small, flexible-molecules which may optionally have a disulfide bridge.

SUMMARY OF THE INVENTION

[0003] It is an object of the present invention to provide peptides which comprise a monosulfide bridge. This thioether bond is also designated as a lanthionine bridge and corresponds with the cystine bridge with the exception that the disulfide bridge is replaced by a monosulfide linkage. Two amino acid residues having the general formula

[0004] are designated to be joined into a lanthionine bridge wherein the linkage of the two amino acids has the meaning —RCH—S—HCR′—, wherein R and R′ respectively represent —H, a lower (C1-C10) alkyl or aralkyl group. In a preferred embodiment R and R′ are H. The amino acid termini of the lanthionine structure are designated as Ala_(L) if R and R′ are H and Thr_(L) when R or R′ are CH3. Other β-substituted lanthionine components are designated as substituted Ala_(L) derivatives, e.g. βethylAla_(L).

DESCRIPTION OF THE RELATED ART

[0005] Thioether bonds of the lanthionine type are known from some fungal toxins and antibiotics, for example from the lantibiotics, as nisin, epidermin, dunamycin or mersacidin. Naturally occurring compounds having the monosulfide bridge always have more than two monosulfide bridges in the molecule.

[0006] M. F. Bean et al. have reported in their article “Identification of a Thioether By-product in the Synthesis of a Cyclic Disulfide Peptide by Tandem Mass Spectrometry” as published in the Proceedings of the 11th American Peptide Symposium, ESCOM, (Leiden 1990, p. 443) on a somatostatin analog wherein the internal disulfide bond has been converted to a thioether link. The somatostatin analog with the putative amino acid sequence Phe-Ala-Phe-Trp-Lys-Thr-Ala-Thr(ol), wherein the two Ala_(L) residues are linked via the thioether bridge, has been described as the by-product which was obtained by the Boc-TFA-preparation of sandostatin analogs. The originally occurring somatostatin derivative has a disulfide bridge.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention will now be described in greater detail with reference to the drawings wherein:

[0008] FIGS. 1-4 describe different syntheses of lanthionine bridged compounds.

DETAILED DESCRIPTION OF THE INVENTION

[0009] It is an object of the present invention to provide analogs of peptide compounds having a monosulfide bridge in the molecule and exhibiting an improved biological activity. The analogs of peptide compounds according to the invention comprise analogs of compounds such as: virus related peptides, such as peptides of HIV, fragments of oncogenes, atrial natriuric peptides, epidermal growth factors, transforming growth factor and fragments thereof, spanolysin, TRP, xenopsin, PGL, PGA, custracean, bactenecin, relaxin, cyclotheonamides, casoxin D, mastoparan, azaline-B, GRF (growth hormaone releasing factor), leucinostatins, didemnins, nazumamides, theonellamides, glicentin, aureobasidins, guanylin, NGF (nerve growth factor), gluten exorphins, paradaxin P-1, neuromedin U (NMU), bactenecins, bacitracin, bombyxin, PTH, PTHrP (para-thyroid hormon-related protein), agelenin, insulin, glucagon, tyrocidins, polymyxins, valinomycin, conotoxins and related neurotoxins, mast cell granulating peptides (MCD), HIV gp41 antigenic peptide 1 or peptide 4, TNF (tumor necrosis factor), RGD-peptides (peptides containing the sequence . . . -Arg-Gly-Asp- . . . ).

[0010] It is an especially preferred embodiment of the present invention the peptide or the fragment thereof having the thioether bond is derived from a molecule having the biological activity which is in the naturally occurring form linear.

[0011] In a preferred embodiment of the present invention the peptide has not more than two monosulfide bridges and in an especially preferred embodiment the peptide has only one monosulfide bridge.

[0012] In a further preferred embodiment of the present invention the naturally occurring molecule has two or more cyclic structures wherein at least one cycle is formed by a thioether bond.

[0013] The compounds of the present invention have a higher biological activity than the corresponding naturally occurring peptides.

[0014] According to the present invention lanthionine-bridged peptides are disclosed having the general formula

[0015] wherein

[0016] R₁ is selected from the group consisting of sequences of 2 to 7 amino acids selected from naturally occurring amino acids and the D-enantiomers thereof and peptidomimetrics;

[0017] R₂ is selected from the group consisting of

[0018] a) —H, C₁₋₈ alkyl, C₇₋₁₂ aralkyl, —HCO, C₂₋₁₈ acyl or C₂₋₁₈ aracyl,

[0019] b) a naturally occurring amino acid or sequence of up to 25 amino acids where the —N terminal —NH₂ group is present or is replaced by C₁₋₈ alkyl, C₇₋₁₂ aralkyl, —HCO, or C₂₋₁₈ acyl, —OH, —H or NHCOR₆, and

[0020] c) peptidomimetics;

[0021] R₃ is selected from the group consisting of

[0022] a) —OH, NH₂;

[0023] b) naturally occurring amino acid or sequence of up to 25 amino acids where the —C terminal —COOH is present or is replaced by —CONH₂ or —CH₂OH;

[0024] c) peptidomimetics; and O

[0025] d) CR₃ may be replaced by CH₂OH; with the proviso that R₁ is not Phe-Trp-Lys-Thr when R₂ is Phe and R₃ is Thr[ol];

[0026] R₄, R₅, R₇ and R₈ are independently hydrogen, cyclohexyl or a substituted or unsubstituted C₁₋₁₀ alkyl; and

[0027] R₆ is an alkyl or aralkyl residue.

[0028] In a preferred embodiments of the present invention R₄, R₅, R₇ and R₈ are independently hydrogen or a methyl group.

[0029] The amino acids can be selected from the group consisting of the naturally occurring amino acids including the L-enantiomers and the D-enantiomers. The group of the naturally occurring amino acids comprises alanine, arginine, asparagine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, histidine, hydroxyproline, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine, β-alanine, γ-aminobutyric acid, betaine, carnitine, citrulline, creatine, ornithine, saccharopine, 3,4-dihydroxyphenylalanine, 5-hydroxytryptophan, thyroxine, homocysteine, S-methylmethionine, penicillamine, pipecolic acid and nalidixic acid.

[0030] A regular peptide has the following structure:

[0031] whereas the retro-inverso structure has the following formula:

[0032] The radicals R₁, R₂ or R₃ can comprise peptidomimetics. Non-natural amino acids which can be used in out invention are identified by the following:

[0033] α-methylated amino acids

[0034] β-mono and dimethylated amino acids

[0035] Tic [7-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid]

[0036] Atc [2-aminotetralin-2-carboxylic acid]

[0037] Aic [2-amino-5-hydroxyimolinane-carboxylic acid]

[0038] Hat [2-amino-6-hydroxytetralin-2-carboxylic acid]

[0039] Nal 2-(1-napththyl)alanine

[0040] ΔPhe dehydro-phenylalanine

[0041] p-x-Phe p-halogeno-phenylalanine

[0042] Me₃Phe 2-dimethyl-β-methyl-phenylalanine

[0043] Examples of dipeptide isosteres which may be utilized are:

[0044] The terto-inverso structure is only one possible modification to the regular peptide structure. The regular structure may be modified by the presence of a thioester structure according to the following formulae:

[0045] In other embodiments, the regular peptide structure may be modified in accordance with the formula:

[0046] where X is

[0047] CH₂—S [methylene thioester]

[0048] CH₂—S(O) [methylene sulfoxide]

[0049] CH₂—O [methylene ether]

[0050] C(S)—NH [thioamide]

[0051] CO—CH₂ [ketomethylene]

[0052] CH₂—CH₂ [carba]

[0053] NH—NH [aza]

[0054] CH₂—NH [methylene amino]

[0055] In a preferred embodiment of the present invention the moiety R₁ of the peptides according to formula (I) comprises a sequence of from 2 to 4 amino acids.

[0056] Preferably the amino acids of the residues R₂ and R₃ are selected from the group of amino acids comprising D-Phe, D-β-Nal, Tyr, TrpNH2, ThrNH2, Thr(ol). Alternatively the substituents R₂ can be H, acyl or aracyl with 1 to 18, preferably 2-12 carbon atoms and R₃ can have the meaning of —OH or —NH₂and O—C—R3 can be replaced by CH₂OH.

[0057] Moreover R₂ and R₃ respectively can be a short amino acid sequence of Pro-Arg-Gly or Pro-Leu-Gly.

[0058] In preferred embodiments of the present invention R₁ is represented by a short amino acid sequence selected from the group consisting of Gly-Phe; Phe-D-Trp-Lys-Thr; Phe-D-Trp-Lys-Val; Tyr-Phe-Gln-Asn, Tyr-lle-GIn-Asn, Tyr-D-Trp-Lys-Val, Gly-Asn-Leu-Ser-Thr, Ser-Asn-Leu-Ser-Thr or Glu-Lys-Asp-Met-Leu-Ser-Ser.

[0059] Although the naturally occurring biologically active molecules contain mainly the L-enantiomers of the amino acids, in one preferred embodiment of the present invention at least one of the amino acid forming the lanthionine bridge is the D-enantiomer.

[0060] Among the especially preferred peptides of the present invention are: lanthionine-enkephalins [SEQ ID NO:1] having the formula

[0061] wherein R₃ is OH or NH₂, lanthionine-somatostatins [SEQ ID NO: 2 to SEQ ID NO: 11] having the general formula

[0062] wherein Xxx=D-Phe, D-β-Nal or Phe; Yyy=Thr, Val; Zzz=TrpNH₂, ThrNH₂ or Thr(ol) with the proviso that Xxx is not Phe when Zzz is Thr(ol) and Yyy is Thr.

[0063] Lanthionine-vasopressin [SEQ ID NO: 12 to SEQ ID NO: 14] having the formula

[0064] wherein Aaa=Phe and Bbb=Arg, Leu or Lys lanthionine-oxytocin [SEQ ID NO: 13 to SEQ ID NO: 15]

[0065] wherein Aaa=lie and Bbb=Leu or Arg.

[0066] Further preferred peptides of the present invention have the formula (VI)

[0067] or the formula (VII)

[0068] Further preferred peptides [SEQ ID NO: 16 to SEQ ID NO: 21] have the general formula (VIII)

[0069] wherein R8 is H, acyl or aracyl, R9 is the fragment 8-32 of human, salmon or eel-calcitonin and Ggg is Gly or Ser.

[0070] In another preferred embodiment of the present invention the peptide has the amino acid sequence of endothelin (Schematic Structure A see below) wherein one or two of the naturally occurring disulfide bridges are replaced by a thioether bond. Therefore the compounds can be shown as described by the schematic structures B [SEQ ID NO: 22], C [SEQ ID NO: 23] and D [SEQ ID NO: 24].

Schematic Structures

[0071] The preferred peptides of the present invention will have sequentially overlapping thioether bonds as shown above, if the peptide has two thioether linkages. This means that in the amino acid sequence one lanthionine-bridge is located between two Ala_(L) residues forming the second lanthionine-bridge.

[0072] The peptides of the present invention can be used as pharmaceutically active compounds. They can therefore be used in pharmaceutical compositions comprising at least one of the peptides of the present invention.

[0073] Pharmaceutical formulations of the peptides comprising the peptides of this invention and a pharmaceutically acceptable carrier are part of this invention. As used herein, the term “pharmaceutically acceptable” carrier encompasses any of the standard pharmaceutical carriers, such as phosphate buffered saline solution, water, emulsions, such as oil/water emulsions, and various types of wetting agents.

[0074] The compounds may be administered intravenously and parenterally using well known pharmaceutical carriers or inert diluents. Oral administration is not preferred because the peptides will tend to be degraded by the enzymes of the alimentary tract.

[0075] In respect to pharmaceutical compositions containing the peptides disclosed herein, carrier and other ingredients should be such that as not to diminish the therapeutic effects of the peptides.

[0076] The pharmaceutical compositions of this invention thus will generally comprise an effective amount of the peptide in a suitable pharmaceutically acceptable carrier such as water or alcohols which may contain fillers, stabilizers, wetting agents, emulsifying agents and dispersing agents to name but a few conventional carriers, adjuvant and excipients which may be employed.

[0077] The injectible compositions are formulated as shown in the art and may contain appropriate dispersing or wetting agents and suspending agents identical or similar to those mentioned above.

[0078] Depending on the nature of the biologically active peptide they can be used in injection solutions, capsules, tablets, ointments, creams, sprays and suppositories.

[0079] A representative example of the peptides of the present invention can be produced according to the following procedure described for the enkephalin [SEQ ID NO: 1]

[0080] The linear peptide chain was assembled on a methylbenzhydrylamine resin using tert-butoxycarbonyl chemistry with the symmetrical anhydride peptide coupling method. A serine residue was preferably incorporated at position 2 and later on converted to dehydroalanine using disuccinimido carbonate. The S-protecting group (fluorenyl methyl) was selectively removed with piperidine.

[0081] A slightly basic milieu, preferably 5% piperidine/dimethylformamide, promoted the Michael addition of the SH-group to the double bond. The amino acid analysis showed 48% conversion of serine. A greater excess of the reagent disuccinimido carbonate would have resulted in an increased yield for these two steps. “Low-high HF cleavage” was used to cleave the peptide from the resin and to remove the protecting groups. Purification of the resultant crude product was achieved by preparative RP-HPLC using a gradient acetonitrile-water elution. The material obtained was further purified and desalted by gel filtration on a Sephadex G-15 column (20% acetic acid/water).

[0082] The product was identified by amino acid analysis and mass spectrometry. Although the Michael addition is not stereoselective, this reaction resulted in only the (2D, 5L) diastereomer. The other diastereomer expected, (2L, 5L) could not be detected in the reaction mixture. Steric hindrance from the solid support near the SH group may be responsible for the stereoselectivity of the addition reaction. The solid phase synthetic approach allows a rapid assembly of lanthionine-bridged cyclopeptides. FIG. 1 shows schematically the process according to the invention.

Abbreviations Used in Peptide Synthesis Section

[0083] Standard abbreviations for amino acids and protecting groups are followed according to the IUPAC-IUB Joint Commission on Biochemical Nomenclature: J. Biol. Chem. 1971, 247, 977.

[0084] Abbreviations used: Acm, amidocarboxymethyl; Boc, ter-butoxycarbonyl; Bzl, benzyl; DCC, N,N′-dicyclohexylcarbodiimide; DCM, dichloromethane; Dha, dehydroalanine; DMF, dimethylformamide; DIEA, N,N-diisopropylethylamine; DSC, disuccinimido carbonate; EtOAc, ethyl acetate; Fm, fluorenylmethyl; Fmoc, fluorenylmethyloxycarbonyl; HOBt, 1-hydroxybenztriazole, Ala_(L), lanthionine; MBHA, methylbenzhydrylamine resin; Pac, phenylacyl; TFA, trifluoroacetic acid; Tmse, trimethylsilylethyl; Z, benzyloxycarbonyl; NCA, N-carboxyanhydrid; Trt, trityl.

Experimental Procedures

[0085] All amino acids were of the L-configuration except as indicated. Protected amino acids were purchased from Bachem, Inc. ACS grade solvents (DCM, DMF, acetonitrile) were purchased from Fisher Scientific and purged with nitrogen, then stored over molecular sieves from Sigma. DIEA (Aldrich) was dried over KOH and distilled from ninhydrin. MBHA resin.HCl (Calbiochem) was swollen in DCM and washed with 5% DIEA/DCM followed by DCM before use. TFA, piperidine, DSC (Aldrich) and DCC (Fluka) were used without further purification. Silica gel for flash chromatography was purchased from Baker.

[0086] Peptides were analyzed on precoated silica gel 60F-254 plates (Merck) using (A) chloroform:methanol:acetic acid, 65:35:1; (B) butanol:acetic acid:water, 4:1:5-upper phase. Compounds were visualized by UV, ninhydrin, chlorine/o-tolidine and KMnO₄ solution. RP-HPLC analyses were performed on a Waters (Model 510 and Waters 484 detector) instrument with a C-18 analytical column. [pages 11-17 are missing from the electronic version of the specification]

[0087] There is, however, another synthetic method for the production of peptides. It is often desirable to have diastereomeric peptide analogs. The use of diastereomeric mixture of lanthionine units can provide appropriate diastereomeric analogs, separable by chromatography (HPLC). By such routes, two (or four) analogs can be prepared by a single synthetic process. The simultaneous application of the benzyloxycarbonyl, t-butyloxycarbonyl and phenacyl (or methyl, trimethylsilylethyl, etc.) groups defines the synthetic strategy of this invention to prepare

[0088] A new application of the PCOR method (Peptide Cyclization on an Oxime Resin) can provide the new cyclic segment containing a defined lanthionine bridge (Scheme 2), where the chain can be elongated at both termini. The process without a lanthionine bridge has been described in more detail by Osapay et al. in J. Am. Chem. Soc. 1990, 112, p. 6046-6051 and Tet. Lett. 1990, 31, p. 6121-6124. The final deprotection step followed by chromatographic purification yields the desired compounds.

[0089] Another highly promising pathway involves the synthesis of two protected intermediates, followed by coupling of the two components in generating an optically pure lanthionine. This is proceeded by the synthesis of both the protected serine β-lactone (Arnold et al. J. Am. Chemn. Soc. 1988, 110, p. 2237-2241) and the protected cysteine. The latter compound acts as a nucleophile in opening the lactone ring at the site of the methylene group (see Scheme 3).

[0090] The β-lactone method is shown in FIG. 2 wherein also the obtainable yields are given.

[0091] Furthermore another route for the synthesis of the protected lanthionine is disclosed where reactions proceed with retention of configuration. This lanthionine derivative is prepared through the ring opening of an aziridine derivative (Wakamiya et al., Bull. Chem. Soc. Jpn., 1982, 55, 3878-3881) by a nucleophile, namely cysteine or any appropriate SH-containing amino acid (Scheme 4).

[0092] The Aziridine method is shown in FIG. 3 wherein also the yields obtainable by said method are given.

[0093] Preparation of Lanthionine-opioids

[0094] As will be shown later, all of the lanthionine opioids synthesized are superactive both at the μ- and δ-receptor. To investigate structural or pharmacochemical aspects of this new class of opioids, analogs can be synthesized in order to carry out bioassays and conformational analyses of the resulting molecules. Various peptidic or peptidomimetic units can be incorporated into cyclic enkephalin and dermorphin-deltorphin structures including:

[0095] The incorporation of methyl group(s) at the β-carbon(s) and effects of chirality at the two main chain units of the lanthionine residue can also be included.

[0096] Thus the synthesis of β-methyl lanthionines and β,β-dimethyl lanthionine results in modifications, which are expected to lead to substantial differences in bioactivity profiles for closely related target molecules. Thus, critical information about the “bioactive conformations” of the analogs can be obtained. In addition, specific residues such as the Gly in

[0097] and the Asp of

[0098] can be modified with natural and unnatural amino acids. This family of opioids are most promising for obtaining novel and clinically useful opioid drugs.

[0099] Lanthionine-somatostatins

[0100] New lanthionine-somatostatin derivatives can be synthesized. First, the cyclic segment with a monosulfide bridge of somatostatin or “key-hexapeptide” (Scheme 2) or other analogs of somatostatin according to the definition of R₁ on a Kaiser-oxime resin has to be prepared. It will be elongated at both termini (D-Phe at the N-terminus and threoninol at the C-terminus) to obtain for example the lanthionine analog of Sandostatin. The same synthetic strategy can be used for the preparation of the lanthionine analog of the native somatostatin tetradecapeptide. Potency and receptor selectivity of both target molecules are promising.

[0101] Lanthionine-calcitonins

[0102] It is possible to incorporate the lanthionine as the replacement of the cysteine-cysteine disulfide bridge in the N-terminal loop. The loop can be prepared via the PCOR method (Scheme 5).

[0103] Human and rat calcitonins: Aaa=Gly

[0104] Salmon and eel calcitonins: Aaa=Ser

Loops of Lanthionine-calcitonin

[0105]

[0106] The elongation at the C-terminus to get the final calcitonin-analog can be performed by normal classical fragment condensations or by the strategy shown later in the paragraph of lanthionine-oxytocin and -vasopressin synthesis (Scheme 6).

[0107] Lanthionine-oxytocins and Lanthionine-vasopressins

[0108] The incorporation of a lanthionine bridge to replace the existing disulfide bridge found in natural oxytocin (OT) and vasopressin (VP) is another example. This can be accomplished by the synthesis of the lanthionine component prior to its incorporation in the peptide sequence (Scheme 6).

[0109] The following examples illustrate the present invention without limiting the scope of the invention thereto.

EXAMPLE 1

[0110] a) Preparation of Z-Tyr(Bzl)-Ser-Gly-Phe-Cys(Fm)-MBHA (1)

[0111] Methyl benzhydrylamine resin (3 g) was reacted with Boc-Cys(Fm)OH (1.0 g, 2.5 mmol) and DCC (0.52 g, 2.5 mmol) in DCM (30 mL) for 3 hr at room temperature in an SPPS vessel. The remaining amino groups were capped by acetylation. The resulting Boc-Cys(Fm)-MBHA resin (substitution level 0.36 mmol/g, based on picric acid titration) was then deprotected with 30% TFA/DCM (v/v) and neutralized with 1% DIEA/DCM (v/v) solution. The peptide chain was then assembled by consecutive addition of the symmetrical anhydrides (2.5 equiv.) of BocPheOH, BocGlyOH, BocSerOH, and ZTyr(Bzl)OH as well as deprotection steps. The completeness of coupling was monitored by the Kaiser test. Coupling of ZTyr(Bzl)OH was repeated with 1 molar equivalent reagent. Yield 1.06 mmol (84%) peptide based on Gly; amino acid analysis; Cys₍₁₎Gly_(1.00)Phe_(0.86)Ser_(1.42)Tyr_(1.22).

[0112] b) Preparation of

[0113] The protected peptidyl MBHA resin (1, 1.06 mmol) in the SPPS vessel was swollen and then suspended in DCM (20 mL). A solution of DSC (387 mg, 1.51 mmol) in acetonitrile (10 mL) was added to the reaction mixture followed by a 5% DIEA/DCM solution (5.22 mL, 1.5 mmol DIEA). The reaction was allowed to proceed for 4 hr, shaking at room temperature in a nitrogen atmosphere. The reaction mixture was drained and the solid phase was washed with DCM (4×). The product (2) was treated with a solution of 20% piperidine/DMF solution (20 mL, v/v, 2×50 min.) and shaken in a 5% piperidine/DMF-DCM solution (40 mL, 1/1, v/v) overnight. The solution phase was drained and the resin was washed with DMF (1×), DCM (2×) and EtOH (2×) and dried. Yield 3.7 g.

[0114] c) Preparation of

[0115] The peptidyl resin (3, 1.0 g) was treated with anhydrous HF (20 mL) at 0ØC in the presence of anisole (1 mL) for 1 hr in a teflon HF apparatus. After removal of volatile components the remaining material was washed with EtOAc (2×20 mL) and the product was extracted with acetic acid followed by 10% acetic acid/water solution. The combined extracts were freeze-dried (yield 200 mg). This material was purified by preparative RP-HPLC on a Vydac C-18 column (1.0×25 cm) eluted with 0.1% TFA in acetonitrile/water. A linear gradient from 15% to 22% acetonitrile over 12 min with a flow rate of 10 mL/min was employed. The appropriate fractions were lyophilized to give a solid product (yield 87 mg). Finally, 30 mg of the product was subjected to gel permeation chromatography (1.5×100 cm Sephadex G-15 eluted with 20% acetic acid). The peptide fractions were pooled and lyophilized. Yield 16 mg (24% calculated for compound 1). RF(A) 0.44; RF(B) 0.49. FAB-MS m/e=557 (M+1). Amino acid analysis Gly_(1.00)Ala_(L)-S-Ala_(L1.1)Phe_(0.99)Tyr_(0.95).

EXAMPLE 2

[0116]FIG. 4 shows schematically the synthesis of lanthionine-enkephalin in solution. Other general methods for chemical synthesis can be followed using mixed anhydrides, carbodiimides, active esters and other coupling procedures. Preferred solvents are CH₂Cl₂ and DMF. Cleavages are following standard selective reactions. Purification follows well-known extractions, precipitations and chromatographic methods.

EXAMPLE 3

[0117] The lanthionine-enkephalin was also synthesized by the preferred Fmoc-NCA-Strategy by using the following steps: (1) Deprotection (20% piperidine/DMF)  7 min 30 mL/min (2) Wash (DMF)  5 min 30 mL/min (3) Coupling (see below) 20 min 30 mL/min (4) Wash (DMF)  5 min 30 mL/min (5) Repeat steps 1-4

[0118] The coupling was performed as follows:

#1:3 eq. 20 min; #2:1 eq.+DIEA 20 min

[0119] (a) Fmoc-Phe-NCA,

[0120] (b) Fmoc-Gly-NCA,

[0121] (c) Fmoc-Ser-OH/DCC,

[0122] (d) Fmoc-Tyr(Bzl)-NCA.

[0123] In the case of peptide chain elongation by the Fmoc-strategy the —SH group of cystein was blocked with a Trt group.

EXAMPLE 4

[0124] The preparation of lanthionine-bridged cyclic peptide fragments is demonstrated by the following preparations:

[0125] resin (1.0 g, 187 μmol peptide on resin) synthesized by regular solid phase synthetic method was swollen in DCM (10 mL) in a solid-phase peptide synthesis vessel. The Boc group was removed with 25% TFA/DCM, shaking the reaction vessel for 30 min. The peptidyl resin was then drained and washed (10 mL/wash) with DCM (2×), i-PrOH (1×), DCM (2×), i-PrOH (1×), and DCM (2×). The amino group was neutralized by treating the peptidyl resin with 5% DIEA in DCM (2×1 min.) and then washing with DCM (2×). The cyclization reaction was then carried out by shaking the peptidyl resin in DCM/DMF (1:1, v/v, 10 mL) in the presence of 10 equiv. AcOH at RT for 72 h. The cyclic peptide product was collected from the reaction vessel by draining and then washing the resin with DMF (3×). These solutions were combined and evaporated to a reduced volume, and then washed with water, 0.1 N HCl, 5% NaHCO₃, and brine. The solvent was then evaporated and the crude product was purified by silica gel flash chromatography (2×20 cm, ethyl acetate-hexanes 1/1). The appropriate fractions were pooled and the solvent was evaporated. The pure solidified product was recrystallized from methanol/ether. Yield 40.5 mg (36.7%); mp 241-244° C. (decomp); R_(F)(EtOAc/hexanes=2/1) 0.42; FAB-MS m/e=590 (MH⁺); theoretical 590.

[0126] Thr(Bzl)-O-oxime resin (100 mg, 6.0 μmol peptide on resin) synthesized by regular solid phase synthetic method was swollen in DCM (1.0 mL) in a solid-phase peptide synthesis vessel. The Boc group was removed with 25% TFA/DCM, shaking the reaction vessel for 30 min. The peptidyl resin was then drained and washed (1.0 mL/wash) with DCM (2×), i-PrOH (1×), DCM (2×), i-PrOH (1×), and DCM (2×). The amino group was neutralized by treating the peptidyl resin with 2.5% DIEA in DCM (2×1 min.) and then washing with DCM (2×). The cyclization reaction was then carried out by shaking the peptidyl resin in DCM/DMF (1:2, v/v, 1.0 mL) in the presence of 10 equiv. AcOH at RT for 72 h. The cyclic peptide product was collected from the reaction vessel by draining and then washing the resin with DMF (3×). These solutions were combined and evaporated and the crude product was purified by RP-HPLC on a Vydac C-18 column (1.0×25 cm) using 0.1% TFA in acetonitrile water. A linear gradient from 50 to 80% acetonitrile over 15 min., with a flow rate of 4 mL/min., was employed. The product was eluted at 61% acetonitrile and lyophilized to give a solid product. Yield 0.9 mg (24%); R_(F)(CHCl₃/MeOH/AcOH=18/1.5/1) 0.54; FAB-MS m/e=1,293 (MH⁺); theoretical 1,293.

EXAMPLE 5

[0127] Comparative examples showing the superior biological activity of compounds with the thioether bond compound compared with the compound having the disulfide bridge:

[0128] Bioassays Using Isolated Organ Preparations

[0129] All of these assays represent standard procedures which have been well described in the literature.

[0130] (1) The GPI (guinea pig ileum) assay will be performed according to a modified version of a procedure first developed by Paton. Male guinea pigs (300-450 g) are killed by a blow on the skull and exsanguinated. A 2-3 cm segment of ileum not less than 10 cm from the ileocecal junction is mounted in a 20 ml organ bath. The bath contains Krebs' solution of the following composition (in millimolar concentrations): NaCl, 150; KCl, 4.3; CaCl2, 1.25; MgCl2, 1.0; NaH2PO4.H2O, 1.7; NaHCO3, 25.0; glucose, 11.0. The temperature is maintained at 37ØC and the solution is bubbled with 95% 02/% C02. A GRASS E 2B electrode is used as anode with the 1.5 cm platinum wire entirely enclosed within the lumen. The other end of the preparation is tied over a piece of stiff polyethylene tubing (4 cm, 2.5 mm O.D) which projects out of the bath solution and is tied to the strain gauge. Another GRASS E 2B electrode is placed about 5 mm from the intestine and parallel to it to achieve transmural stimulation. Single pulses of 4 msec during are delivered by a Harvard apparatus stimulator at a frequency of 10 min-1. Voltages in the range from 3 to 6 V are applied in order to obtain maximal response. Isometric contractions of the ileum are recorded via a Harvard isometric force transducer on a Harvard apparatus biograph which has been calibrated to produce a pen displacement of 1 cm per tension change of 1 g. The results are standardized by expressing the reduction in tension obtained at each dose level as a percentage of the mean tension produced by at least ten preceding control stimulations. Semilogarithmic plots of percent inhibition as a function of peptide concentration permit the determination of IC50-values which are taken as the intercept of 50% inhibition.

[0131] (2) The MVD (mouse vas deference) assay was performed essentially as described by Henderson. Briefly, adult, male albino mice (Swiss Webster 30-50 g) are killed by cervical dislocation and the vas deferentia are dissected out. After removal of extraneous fat and connective tissue, the vas is stripped of its associated blood vessel and the somen is gently expressed from the lumen. The vas is then mounted under 0.5 g tension in a 5 ml organ bath containing warmed (37° C.), oxygenated (95% 02, 5% CO2%)), Mg2+-free Krebs solutions of the following composition [mM]: NaCl, 118; CaCl2, 2.54; KCl, 4.75; KH2PO4, 1.19; NaHCO3, 25; glucose, 11; L-tyrosine, 0.2. A modified Harvard apparatus stimulator is used to deliver repetitive field stimulation through platinum wire ring electrodes at the top and bottom of the bath, consisting of twin, rectangular pulses (80 V, 0.15 Hz, 10-ms delay, 1.0-ms duration). Contractions of the muscle are recorded via a Hewlett-Packard Model FTA-I-I force transducer connected to a Hewlett-Packard 7702B recorder. Determination of the reduction in the twitch height at various doses permits the construction of log dose-response curves and the determination of IC50-values. TABLE 1 Inhibitory Potency and Selectivity of Lanthionine and Disulfide-Bridged Enkephalin Analogs in GPI (guinea pig ilium) and MVD (mouse vas deferens) Bioassays Adequate to μ and δ Receptor Binding Affinities, Respectively, and Rat Hot-plate in vivo Test GPI MVD GPI/MVD in vivo δ-Binding^(a) Structures IC₅₀ (nM) IC₅₀ (nM) IC₅₀ (nM) IC₅₀ (nM) IC₅₀ (nM) Tyr-c[D-Ala_(L)-Gly-Phe-L-Ala_(L)]-NH₂   0.62  0.54   1.13    0.11     7 Tyr-c[D-Ala_(L)-Gly-Phe-D-Ala_(L)]-NH₂   1.67  1.67   1.0    0.11 N/D Tyr-c[D-Ala_(L)-Gly-Phe-L-Ala_(L)]-OH   0.45  0.16   2.85    0.19     1 Tyr-c[D-Cys-Gly-Phe-L-Cys]-NH₂   1.51  0.76   1.99    0.24 N/D Tyr-c[D-Cys-Gly-Phe-D-Cys]-NH₂   0.78  0.30   2.61    0.29 N/D Tyr-c[D-Cys-Gly-Phe-L-Cys]-OH   3.06  0.19  16.1 N/D N/D Tyr-c[D-Val_(L)-Gly-Phe-L-Ala_(L)]-OMe  88.1  3.65  24.1 N/D N/D Tyr-c[D-Val_(L)-Gly-Phe-L-Ala_(L)]-OH  39.0  0.27  143 N/D N/D Tyr-c[D-Val_(L)-Gly-Phe-D-Ala_(L)]-OH  535  1.51  354.3 N/D N/D Tyr-c[D-Pen-Gly-Phe-D-Pen]-OH 7300  4.1 1800   130    78 Tyr-Gly-Gly-Phe-Leu  246  11.4  21.6 >180^(b) N/D Morphine  58.6 644.0   0.09    15 >1000

[0132] Table 1 shows that the compound according to the invention which has a thioether bond is in both methods, particularly in the most relevant GPI test more potent than the corresponding —S—S— compound.

EXAMPLE 6

[0133] Using protocols described by Schiller et al., Biochem. Biophys. Res. Commun. 1983, 115, p. 864-870, we compared the half-lives of three compounds: Leu⁵-enkephalin. As indicated in Table 2, the lanthionine-enkephalin is much more stable than the other two compounds. TABLE 2 Enzymatic Degradation of Enkephalin Analogs Analog t_(1/2) (min) Lanthionine-enkephalin 1223 Disulfide-enkephalin 332 Leu⁵-enkephalin 30

EXAMPLE 7

[0134] The lanthionine opioid is highly active in the in vitro and in vivo tests (Table 3). In vivo bioactivity was determined using the rat hot plate test after intrathecal dosages.

[0135] shows 37 times higher bioactivity than morphine and twice the activity of DCLCE (Table 3). In the same tests, [Leu⁵]-enkephalin shows only 40-50% of full agonistic activity after 100 pg dosage in the in vitro assays using GPI and MVD preparations. The lanthionine opioid exhibits 400 times greater bioactivity at the GPI (μ-receptor) and 20 times greater bioactivity at the MVD (δ-receptor) than [Leu⁵]-enkephalin. These values are higher than those of its disulfide bridged counterpart, DCDCE. The lanthionine analog does not show a preference for the μ- or the δ-receptor. The IC₅₀ ratio (MVD/GPI) is 0.9. TABLE 3 Potencies of Lanthionine and Disulfide-bridged Somatostatin Analogs to Inhibit Radioligand Binding to Cloned somatostatin Receptors [SSTR2 and SSTR4]* Binding Constants [IC₅₀(nM)] SSTR2 SSTR4 Structures Receptor Receptor H-D-Phe-c[Ala_(L)Phe-D-Trp-Lys-Thr-Ala_(L)]-OH 0 500 H-D-Phe-c[Ala_(L)Phe-D-Trp-Lys-Thr-Ala_(L)]-Thr-ol 5.00 1.00 H-D-Phe-c[Ala_(L)Phe-D-Trp-Lys-Thr-Ala_(L)]-Thr- 10.0 0.50 NH₂ Sandostatin H-D-Phe-c[Ala_(L)Phe-D-Trp-Lys-Thr-Ala_(L)]-Thr-ol 2.4 0.57 c[Ala_(L)3, Ala_(L)14]-Somatostatin-14 0.3 — Somatostatin-14 0.28 0.86

[0136] Although the lanthionine-enkephalins according to the invention possess superactivity, they do not seem to have high receptor selectivity. To imporoive thier selectivity, it is possible to introduce one or more alkyl (methy) group(s) in β position(s) of the lanthionine segment.

[0137] In this case at least one of R₄, R₅, R₇, and R₈ may be alkyl (methyl).

[0138] The two lanthionine octapeptides in Scheme 3 are highly potent on both SSTR2 and SSTR4 receptors. They have similar affinities to SSTR4 as sandostatin but their affinities for SSTR2 are slightly decreased. By comparing the IC₅₀ values for SSTR2/SSTR4 receptors, the disulfide and monosulfide bridged octapeptide alcohols are practically the same. On the contrary, the octapeptide amide has a four times higher selectivity for SSTR4 because its affinity to SSTR2 is decreased. The lanthionine analog of SRIF 14 shows the same binding affinity to SSTR2 as the natural disulfide bridged peptide.

[0139] Preparation A

[0140] Preparation of Lanthionine Building Blocks for Syntheses of Lanthionine Peptides

[0141] 1) Z-Ala_(L)(Boc-Ala_(L)-OPac)-OH

[0142] Benzyloxycarbonyl-serine lactone (1.1 g, 5 mmol) and t-butyloxycarbonyl-cysteine phenacyl ester (2.04 g, 6 mmol) are dissolved in DMF (25 ml) and cesium carbonate (0.97 g, 3 mmol) is added to the solution. The reaction mixture is stirred at room temperature for one hour in nitrogen atmosphere, the solvent is removed under reduced pressure. The residue is dissolved in a mixture of 10% NaHSO₄ and ethyl acetate. The organic phase is washed with water, dried with sodium sulfate and the solvent is removed under reduced pressure. The crude product is purified on silica gel column with a solvent system of chloroform-methanol (10/1, v/v). The fractions containing the product are combined and the solvent removed. The product is a white foam. Yield: 1.46 g (52%); R_(f)(CHCl₃/MeOH-10/1)=0.20; FAB-MS (m/e)=561 (MH)⁺.

[0143] 2) Z-Ala_(L) (Trt-Ala_(L)-OMe)-OH

[0144] The compound can be prepared according to the literature: Photaki, I., Samouilidis, I., Caranikas, S. and Zervas, L. J. Chem. Soc. Perkin I., (1979) 2599.

[0145]3) Z-Ala_(L)(BOC-Ala_(L)-OMe)-OPac

[0146] Benzyloxycarbonyl-aziridine phenacyl ester (111.7 mg, 0.33 mmol) and t-butyloxycarbonyl-cysteine methyl ester (106.1 mg, 0.45 mmol) are dissolved in DCM (2 ml). BF₃.Et₂O (2 drops) is added to the solution at room temperature, and the reaction stirred under nitrogen. After five days, the solvent is removed under reduced pressure. The residue is then dissolved in ethyl acetate, and washed sequentially with 10% NaHCO₃, water, and brine. Ethyl acetate layer is dried over magnesium sulfate and then removed under reduced pressure. The crude product is purified on a silica gel column with a solvent system of ethyl acetate-hexane (1:2, v/v). Compound is a white powder from ethyl acetate-hexane. Yield: 82 mg (43.2%); m.p. 93.5-94.5° C; Rf(EtOAc/Hexane=1/2)=0.3; FAB-MS (m/e)=575(MH)+.

[0147] Found: C, 58.45; H, 5.99; N, 4.82; S, 5.46%.

[0148] Calculated: C, 58.51; H, 5.98; N, 4.88; S, 5.58%.

[0149] Preparation B

[0150] Syntheses of Lanthionine-Somatostatin Analogs

[0151] 1) Z-c[Ala_(L)Phe-D-Trp(For)Lys(Tos)Thr(Bzl)Ala_(L))-OMe

[0152] BocThr(Bzl)-oxime resin (5.4 g, substitution level: 0.26 mmol/g based on picric acid titration) was deprotected with 25% TFA/DCM (v/v) and neutralized by 5% DIEA/DCM (v/v). The peptide chain was then assembled by consecutive addition of 2.5 equiv. BOB ester of BocLys(Tos)OH, Boc-D-Trp(For)OH, BocPheOH and ZAla_(L)(TrtAla_(L)OMe)OH as well as deprotection steps. After the Trt group was removed with 25% TFA/DCM (V/V) the peptidyl resin was washed and neutralized according to the standard oxime resin protocol. The cyclization reaction was carried out by shaking the peptidyl resin in DMF-DCM (100 mL, 2/1 v/v) in the presence of 10 equiv. of acetic acid at room temperature for 24 hr. The cyclic peptide product was obtained from the filtrate of the reaction mixture: The solvent was removed in reduced pressure and the product was precipitated from a mixture of DMF-water.

[0153] Yield: 1.07 g (65%), R_(F)(CHCl₃/MeOH/AcOH)=0.79, RP-HPLC: R_(T)=23.5 min (Vydac C-18 semipreparative column; eluent: acetonitrile-water mixture containing 0.1% TFA; conditions: linear gradient from 30 to 90% of acetonitrile during 30 min.). FAB-MS: m/e 1.173 (M+H⁺).

[0154] 2) Z-D-Phe-c[Ala_(L)Phe-D-Trp(For)Lys(Tos)ThrAla_(L))-OMe

[0155] The protected hexapeptide (1, 1.0 g, 0.85 mmol) was partially deprotected with 33% HBr/AcOH (6 mL, v/v) at 0° C. for 10 min and at room temperature for 50 min. Abs. ether (50 mL) was given to the reaction mixture and kept at 0° C. for 1 hr. The product was filtered and dried. Yield: 0.92 g, R_(F)(EtOAc/BuOH/AcOH/H₂O)=0.93, Z-D-PheOH (3.2 g, 3.29 mmol) and HOBt.H₂O (0.89 g, 6.58 mmol) were dissolved in DMF (5 mL), cooled to 0° C. and EDC 0.69 g, 3.62 mmol), DIEA (1.91 ml, 11.0 mmol) and the HBr salt of the above peptide (0.91 g, 0.82 mmol) were given to the reaction mixture. After being stirred for 1 hr at 0° C. and at room temperature for overnight the product was precipitated by addition of water. The crude product was filtered, washed with 0.5 N HCl, 5% NaHCO₃ and water, then recrystallized from DMF-ether. Yield: 0.9 g (86%), RP-HPLC: R_(T)=15.3 min (same column and eluent written above; conditions: linear gradient from 50 to 75% of acetonitrile during 20 min). FAB-MS: m/e 1,231 (M+H⁺).

[0156] 3) Z-D-Phe-c[Ala_(L)Phe-D-TrpLys(Tos)ThrAla_(L))-ThrNH₂

[0157] The protected heptapeptide (2, 0.3 g, 0.24 mmol) in DMF (6 mL) was stirred with N₂H₄.H₂O (0.6 mL) overnight under nitrogen atmosphere. The solvent was partly removed under reduced pressure then water was added to the reaction mixture. The precipitated product was filtered and dried. Yield: 220 mg, RP-HPLC: R_(T)=10.9 min (same conditions as described for compound 2). FAB-MS; m/e 1,204 (M+H⁺). To a solution of this material (180 mg), 0.15 mmol) in DMF (2 mL) cooled to −15° C., 4N HCl/dioxane (0.225 mL) and t-butyl nitrite (0.042 mL 0.35 mmol) were added. After being stirred for 25 min at the same temperature HCl.H-Thr-NH₂ (0.139 g, 0.9 mmol) and DIEA (0.195 mL, 1.25 mmol) were added to the reaction mixture. After a further 1 hr stirring at −15° C. and 48 hr at 4° C. the mixture was diluted with water. The precipitated product was filtered, washed with 1N HCl, 5% NaHCO₃ and dried. The crude material was recrystallized from DMF-ether. Yield: 192 mg, RP-HPLC: R_(T)=21.5 min (same column and eluent written above; conditions: linear gradient from 50 to 100% of acetonitrile during 25 min). FAB-MS; m/e 1,290 (M+H⁺) and 1,312 (M+Na⁺).

[0158] 4) H-c[Ala_(L)Phe-D-TrpLysThrAla_(L))-OH

[0159] The protected hexapeptide (1, 58.6 mg, 0.05 mmol) was dissolved in hexamethylphosphoramide (1.25 mL) cooled to 15° C., 0.5 N NaOH (0.5 mL) was added dropwise over 30 min. The reaction mixture was stirred for further 30 min at this temperature, then and diluted with water (15 mL) acidified to pH 2.5 with 2.5 N HCl at 0° C. After being kept for 1 hr at this temperature the product was filtered, dried and recrystallized from DMF-ether. Yield: 110 mg, RP-HPLC: R_(T)=13.7 min (same conditions as described for compound 2). FAB-MS: m/e 1,131 (M+H⁺). This material was deprotected with sodium (23 mg) in liquid ammonia (40 mL). After removal of the ammonia the rest was dissolved in water (2 mL) and the pH was adjusted to 5 with acetic acid. The solution was subjected to gel permeation chromatography (1.5×75 cm Sephadex G-10 eluted with 10% acetic acid) followed by RP-HPLC purification on a Vydac C-18 semipreparative column eluted with isocratic 22% acetonitrile/water containing 0.1% TFA. The pure peptide fractions were pooled and lyophilized.

[0160] Yield: 19 mg (51%), RP-HPLC: R_(T)=14.0 min (conditions above). FAB-MS; m/e 753 (M+H⁺).

[0161] 5) H-D-Phe-c[Ala_(L)Phe-D-TrpLysThrAla_(L))-ThrNH₂

[0162] The protected octapeptide (3, 100 mg, 0.077 mmol) was deprotected by sodium (46 mg) in liquid ammonia (75 mL). After removal of the ammonia the residue was desalted by gel permeation chromatography (1.5×75 cm Sephadex G-10 eluted with 10% acetic acid) followed by RP-HPLC purification on a Vydac C-18 semipreparative column. The pure peptide fractions were pooled and lyophilized.

1 24 1 5 PRT Artificial Sequence Cys-Ser lanthionine bridge 1 Tyr Cys Gly Phe Ser 1 5 2 8 PRT Artificial Sequence Cys-Ser lanthionine bridge 2 Phe Cys Phe Trp Lys Thr Ser Thr 1 5 3 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 3 Xaa Ala Ala Cys Phe Trp Lys Thr Ser Thr 1 5 10 4 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 4 Xaa Ala Ala Cys Phe Trp Lys Thr Ser Thr 1 5 10 5 8 PRT Artificial Sequence Cys-Ser lanthionine bridge 5 Phe Cys Phe Trp Lys Thr Ser Trp 1 5 6 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 6 Xaa Ala Ala Cys Phe Trp Lys Thr Ser Trp 1 5 10 7 8 PRT Artificial Sequence Cys-Ser lanthionine bridge 7 Phe Cys Phe Trp Lys Val Ser Trp 1 5 8 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 8 Xaa Ala Ala Cys Phe Trp Lys Val Ser Trp 1 5 10 9 8 PRT Artificial Sequence Cys-Ser lanthionine bridge 9 Phe Cys Phe Trp Lys Val Ser Thr 1 5 10 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 10 Xaa Ala Ala Cys Phe Trp Lys Val Ser Thr 1 5 10 11 10 PRT Artificial Sequence Cys-Ser lanthionine bridge 11 Xaa Ala Ala Cys Phe Trp Lys Val Ser Thr 1 5 10 12 9 PRT Artificial Sequence Cys-Ser lanthionine bridge 12 Cys Tyr Phe Gly Asn Ser Pro Arg Gly 1 5 13 9 PRT Artificial Sequence Cys-Ser lanthionine bridge 13 Cys Tyr Ile Gly Asn Ser Pro Arg Gly 1 5 14 9 PRT Artificial Sequence Cys-Ser lanthionine bridge 14 Cys Tyr Phe Gly Asn Ser Pro Leu Gly 1 5 15 9 PRT Artificial Sequence Cys-Ser lanthionine bridge 15 Cys Tyr Ile Gly Asn Ser Pro Leu Gly 1 5 16 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and second Ser 16 Cys Gly Asn Leu Ser Thr Ser Val Leu Gly Lys Leu Ser Gln Glu Leu 1 5 10 15 His Lys Leu Gln Thr Tyr Pro Arg Thr Asp Val Gly Ala Gly Thr Pro 20 25 30 17 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and third Ser 17 Cys Ser Asn Leu Ser Thr Ser Val Leu Gly Lys Leu Ser Gln Glu Leu 1 5 10 15 His Lys Leu Gln Thr Tyr Pro Arg Thr Asp Val Gly Ala Gly Thr Pro 20 25 30 18 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and second Ser 18 Cys Gly Asn Leu Ser Thr Ser Met Leu Gly Thr Tyr Thr Gln Asp Phe 1 5 10 15 Asn Lys Phe His Thr Phe Pro Gln Thr Ala Ile Gly Val Gly Ala Pro 20 25 30 19 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and third Ser 19 Cys Ser Asn Leu Ser Thr Ser Met Leu Gly Thr Tyr Thr Gln Asp Phe 1 5 10 15 Asn Lys Phe His Thr Phe Pro Gln Thr Ala Ile Gly Val Gly Ala Pro 20 25 30 20 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and third Ser 20 Cys Ser Asn Leu Ser Thr Ser Val Leu Gly Lys Leu Ser Gln Glu Leu 1 5 10 15 His Lys Leu Gln Thr Tyr Pro Arg Thr Asn Thr Gly Ser Gly Thr Pro 20 25 30 21 32 PRT Artificial Sequence Cys-Ser lanthionine bridge between Cys and second Ser 21 Cys Gly Asn Leu Ser Thr Ser Val Leu Gly Lys Leu Ser Gln Glu Leu 1 5 10 15 His Lys Leu Gln Thr Tyr Pro Arg Thr Asn Thr Gly Ser Gly Thr Pro 20 25 30 22 21 PRT Artificial Sequence -S- bridge between Cys and fourth Ser and -S-S- bridge between second and third Ser 22 Cys Ser Cys Ser Ser Leu Met Asp Lys Glu Cys Val Thr Phe Ser His 1 5 10 15 Leu Asp Ile Ile Trp 20 23 21 PRT Artificial Sequence -S- bridge between the second Cys and fourth Ser and -S-S- bridge is between first and third Cys 23 Cys Ser Cys Ser Ser Leu Met Asp Lys Glu Ser Val Tyr Phe Cys His 1 5 10 15 Leu Asp Ile Ile Trp 20 24 21 PRT Artificial Sequence -S- bridge between first Cys and fifth Ser and -S- bridge between second Cys and fourth Cys 24 Cys Ser Cys Ser Ser Leu Met Asp Lys Glu Ser Val Tyr Phe Ser His 1 5 10 15 Leu Asp Ile Ile Trp 20 

We claim:
 1. Biologically active peptides having improved biological activity selected from the group consisting of lanthionine-bridged biologically active peptides of the formula

wherein R₁ is selected from the group consisting of a sequence of 2 to 7 amino acids selected from naturally occurring amino acids and the D- enantiomers thereof and peptidomimetrics; R₂ is selected from the group consisting of —H, C₁₋₈ alkyl, C₇₋₁₂ aralkyl, —HCO, C₂₋₁₈ acyl or C₂₋₁₈ aracyl, a naturally occurring amino acid or sequence of up to 25 amino acids where the —N terminal —NH₂ group is present or is replaced by C₁₋₈ alkyl, C₇₋₁₂ aralkyl, —HCO, or C₂₋₁₈ acyl —OH, —H or —NHCOR₆, and peptidomimetics; R₃ is selected from the group consisting of —OH, NH₂; a naturally occurring amino acid or a sequence of up to 25 amino acids where the —C terminal —COOH is present or is replaced by —CONH₂ or —CH₂OH; and peptidomimetics; and

with the proviso that R₁ is not Phe-Trp-Lys-Thr when R₂ is Phe and R₃ is Thr[ol]; R₄ is hydrogen, cyclohexyl or a substituted or unsubstituted C₁₋₁₀ alkyl; R₅ is hydrogen, cyclohexyl or a substituted or unsubstituted C₁₋₁₀ alkyl; R₆ is an alkyl or aralkyl residue; R₇ is hydrogen, cyclohexyl or a substituted or unsubstituted C₁₋₁₀ alkyl; and R₈ is hydrogen, cyclohexyl or a substituted or unsubstituted C₁₋₁₀ alkyl.
 2. The peptide of claim 1 wherein R₁ is selected from the group consisting of from 2 to 4 amino acids.
 3. The peptide of claim 1 wherein R₂ is a sequence of 3 amino acids; and R₃ is a sequence of 3 amino acids.
 4. The peptide of claim 3 wherein R₂ and R₃ are individually selected from the group consisting of Pro-Arg-Gly and Pro-Leu-Gly.
 5. The peptide of claim 1 where R₂ and R₃ are each individually selected from the group consisting of a sequence of 3 amino acids where the —N terminal —NH₂ is replaced by —OH, —H or —NHCOR6.
 6. The peptide of claim 1 wherein R₂ is selected from the group consisting of D-Phe, D-β-Nal, Tyr, TrpNH₂, ThrNH₂, Thr(ol), Pro-Arg-Gly, Pro-Leu-Gly, —H, C₁₋₈ alkyl, C₇₋₁₂ aralkyl, C₂₋₁₈ acyl and C₂₋₁₈ aracyl R₃ is selected from the group consisting of D-Phe, D-β-Nal, Tyr, TrpNH₂, ThrNH₂, Thr(ol), Pro-Arg-Gly or Pro-Leu-Gly, —OH, —NH₂, and alkylated amino acids; and

 may be replaced by CH₂OH with the proviso that R₁ is not Phe-Trp-Lys-Thr when R₂ is D-Phe and R₃ is Thr(ol).
 7. The peptide of claim 1 wherein R₁ is selected from the group consisting of Gly-Phe, Phe-D-Trp-Lys-Val, Phe-D-Trp-Lys-Thr, Thr-D-Trp-Lys-Val, Tyr-Phe-Gln-Asn, Tyr-Ile-Gln-Asn, Gly-Asn-Leu-Ser-Thr, Ser-Asn-Leu-Ser-Thr and Glu-Lys-Asp-Met-Leu-Ser-Ser.
 8. The peptide of claim 1 wherein R₄, R₅, R₇, R₈ are individually selected from the group consisting of —H, —CH₃ and cyclohexyl.
 9. The peptide of claim 1 wherein R₂ and R₃ are individually selected from the group consisting of D-Phe, D-β-Nal, Tyr, TrpNH₂, and Thr(ol).
 10. Lanthionine bridged peptide analogs of naturally occurring peptides of claim 1 having a biological activity greater than that of the naturally occurring peptide.
 11. Lanthionine bridged enkephalin analog peptides of claim 1 having a biological activity greater than that of enkephalin.
 12. The peptides of claim 1 having no more than two lanthionine bridges.
 13. The peptides of claim 1 having one lanthionine bridge.
 14. Peptides of claim 1 having the general formula


15. Peptides of claim 1 having the general formula

wherein Xxx is selected from the group consisting of D-Phe and D-β-Nal; Yyy is selected from the group consisting of Thr and Val, and Zzz is selected from the group consisting of TrpNH₂, ThrNH₂ and Thr(ol) with the proviso that Xxx is not D-Phe when Zzz is Thr(ol).
 16. Peptides of claim 1 having the general formula


17. Peptides of claim 1 having the general formula

wherein Aaa is selected from the group consisting of Phe and Ile and Bbb is selected from the group consisting of Arg and Leu.
 18. Peptides of claim 1 having the general formula

wherein Ggg is selected from the group consisting of Gly and Ser, R₂ is —H, C₁₋₁₈ acyl and aracyl, and R₃ is selected from the group consisting of fragment 8-32 of human, salmon and eel-calcitonin.
 19. Naturally occurring peptides having a linear structure cyclized by a thioether bond to form a peptide of claim
 1. 20. Peptides having the amino acid sequence of endothelin or an endothelin analog modified to a peptide of claim 1 wherein 1) at least one of the disulfide bridges in the endothelin or endothelin analog has been replaced by a thioether bond and 2) the rings are sequentially overlapping.
 21. A pharmaceutical composition comprising a peptide of claim 1 in a physiologically acceptable carrier.
 22. In a process for the preparation of a peptide according to claim 1 using an appropriate combination of solid-phase peptide synthesis and/or classical synthesis methods, the improvement consisting of using at least one peptide fragment containing a moiety which is cyclized either attached to the resin used or after cleavage from the resin to the desired lanthionine-bridged cyclic peptide fragment which can optionally be elongated at the —N and/or —C terminal to form the final peptide by fragment condensation or step by step synthesis.
 23. A process according to claim 22 for the preparation of a peptide according to claim 1 wherein the peptide fragments containing the moiety to be cyclized are assembled on an appropriate resin using tert-butoxcarbonyl-chemistry with any peptide coupling method, serine is incorporated at the desired place, which is then converted to dehydroalanine using disuccinimido carbonate, the S-protecting group attached to the cystine coupled the desired place is selectively removed, the Michael addition of the SH group to the double bond is promoted by a slightly basic milieu, and the peptide and the other protecting groups are cleaved from the resin by treatment with HF.
 24. The process according to claim 23 wherein the peptide chain is assembled at any appropriate resin using the Fmoc-strategy with any usable coupling agent, intermediately using cleavage of the Fmoc-protecting-group by the piperidine-method, wherein the cleavage of the acid labile S-protecting group is carried out by any appropriate acid or reagent. 